Sync / Async ambiguity in StorageTaskCallback callbacks

[Parsonswy]

No explicit decision was ever made on whether StorageTaskCallbacks should be executed on an async thread, or if the callback needs to run on the Bukkit thread so it can interact with API functions. Given that storage tasks can be invoked synchronously or asynchronously, this is not as trivial as calling .runTask() vs .runTaskAsync() because a storage task that gets invoked synchronously might be expected to block until the provied StorageTaskCallback is complete, but using runTask() would cause a pre-mature return before all post-processing was complete. We need to also be able to detect and distinguish, either by argument or by convention, immediately invoked synchronous callback tasks and deferred, synchronous tasks.

[Parsonswy]

https://www.spigotmc.org/threads/determine-if-method-is-running-async.69651/

https://hub.spigotmc.org/javadocs/spigot/org/bukkit/Bukkit.html#isPrimaryThread()

[Parsonswy]

CompletableFutures are strictly better here and I don't remember why I didn't choose to use the before. Migrating Storage tasks to use CompletableFutures.